The present invention relates to power feeding for an optical transmission system. More particularly, it relates to an apparatus and method for the compensation of fluctuations in the power feed apparatus for an optical transmission system.
Optical transmission systems such as submarine optical communications systems transmit light signals over long distances. Due to optical attenuation effects in the optical transmission lines, it is usually necessary to boost the light signals using repeaters. In order to function, repeaters require an electrical power feed. In addition, there are other devices associated with the optical transmission line which require an electrical power feed.
Typically, the power feed is provided to the repeaters and other devices by a power feed conductor which forms part of a cable including both the conductor and the optical transmission line. The power itself is provided by a high voltage (typically around 8 kV to 20 kV) DC power converter. In effect, this acts as a constant current source, providing a current of around 1A, for example. The repeaters have quite stringent power demands in order to function satisfactorily. For this reason, the current in the power feed conductor should be kept as constant as possible.
The DC power converter is usually located in a terminal on land for easy access and maintenance. For submarine optical transmission lines, the cable must extend between the sea and land to connect to the DC power converter.
Clearly, fluctuations or perturbations in the voltage applied across the repeaters and/or in the current flowing through the repeaters can have a deleterious effect on the transmission of optical data signals. Such fluctuations can arise due to electromagnetic interference, giving rise to induced currents flowing in the power feed conductor.
Similarly, fluctuations can arise due to a change in the local earth potential at or near to the DC power converter. Due to its high voltage output, the DC power converter is usually earthed for safety reasons. It is possible for the local earth potential to change close to the earth plate connected to the DC power converter, and this phenomenon is known as “earth potential rise”.
To try to avoid these fluctuations, the terminals for housing the DC power converters are usually located close to the sea. In addition, electromagnetically “clean” locations are usually selected for the terminals and for the cable extending from the terminals to the sea.
Typically sources of electromagnetic interference are AC power feeders for transmitting industrial power. As is well known, the frequency of the AC power is usually 50 or 60 Hz.
Increasingly, clean electromagnetic locations are difficult to find. In addition, there is greater demand for the terminals to be located further inland than has previously been the case. This increases the length of the land section of the cable and so increases the likelihood of inductive interference at industrial frequencies due to AC power feeders located near to the cable.
Classical shielding measures are often ineffective. Such shielding measures tend to be effective for high frequency (e.g. radio frequency) interference but a very low resistance screen would be necessary to achieve a sensitive screening effect at 50 Hz or 60 Hz. The drawback would be to allow large industrial earth currents to flow through the screen with the risk of creating further interference.
Limitation of the current disturbances by increasing the system impedance with a series inductance can be attempted, but this solution is not suitable for high magnitude 50 Hz or 60 Hz interference. For an effective limitation, a large inductance would be necessary but this could jeopardise the stability of the control loop for the power feed and could lead to unwanted resonance with the high capacitance of the long submarine cable.
Symmetrical cable arrangements or twisted power cables are only effective for low magnitude disturbances. Safety rules do not permit the power feed to operate as a totally floating source, and the low voltage terminal potential has to be limited by a voltage protection device (usually at less than 100 volts).
Conventional power feed equipment current control loops are not sufficient to limit the disturbances discussed above. Such current control loops are necessarily phase shifted and their gain is limited by stability criteria.